1. Field of the Invention
This invention relates to a detection panel and method for acoustic holography.
2. Description of Related Art
The Application of holographic concepts to acoustic waves has led to a new and powerful method for ultrasonic detection and imaging. Acoustic holography is now currently used for a variety of applications from anti-submarine warfare to medical diagnosis.
The principle is quite similar to optical holography in that a reflected acoustic signal from an object is caused to interfere with a local source to form an acoustic hologram. Real time holographic viewing can be achieved by causing the acoustic image of the reflected beam to fall on an array of transducers providing signals which modulate the index of refraction of an electro-optic crystal allowing image reconstruction with a coherent beam of light (see Chapter 12 of the book "Acoustical Holography", Vol 1, published by the Plenum Press, New York, 1969).
The known system is represented in FIG. 1 and consists of a sound source 1 comprising a piezoelectric or magnetostrictive crystal oscillating at the frequency of a signal generator 2, a detector array 3 of piezoelectric or magnetostrictive elements, electronic signal processing circuits 4 and 5, a cathode ray tube 6 with an electro-optic crystal target, a coherent light source 7, optical data processing means 8, a TV camera 9, a TV monitor 10 and a recorder 11, the whole operating under the control of timing signals 12. The efficiency of signal detection as well as image resolution is very much dependent upon the characteristics of the receiving array of detectors. Various detection principles can be used including vibrating membranes in an electromagnetic field, variable capacitance, magnetorestriction and piezoelectricity. Piezoelectric ceramics are by far the most sensitive detectors and are widely used for ultrasonic detection. A representative value of the sensitivity is 1 V/.mu.bar. The resolution of the image is limited by the size of the elements of the array which are generally equal to the wavelength .lambda. of the acoustic signal.